1. Field of the Invention
The present invention relates to a nonaqueous electrolyte secondary battery.
2. Description of the Related Art
As a lithium ion secondary battery, a battery having an arrangement using a lithium composite oxide as a positive electrode active material and a carbonaceous material, which allows doping and undoping of lithium ions, as a negative electrode active material is known. This lithium ion secondary battery is expected to be used as a superior power supply because the battery is small and light, the voltage of a single cell is high, and a high energy density can be obtained.
The conventional lithium ion secondary battery realizes a high energy density by using highly densified positive and negative electrodes. However, in this lithium ion secondary battery including the highly densified positive and negative electrodes, the initial characteristics, particularly, the discharge characteristics when a large electric current is discharged are unstable, and the charge/discharge cycle characteristics deteriorate.
Jpn. Pat. Appln. KOKAI Publication No. 11-86870 discloses a positive electrode in which linear grooves are formed in a depolarizing mix layer formed on the main surface of a current collector and containing a material which participates in an electrochemical reaction, such that the two ends of each groove open at the edges of the depolarizing mix layer. The principal object of this invention is, if the internal temperature of the battery rises due to a shortcircuit or an abnormal electric current and the electrode material or electrolyte gasifies, to rapidly move the gas outside the electrode through the grooves. A reduction in liquid injection time is also described as a secondary effect of the grooves. However, this invention does not mention, e.g., the selection of the form of the grooves in relation to the battery characteristics such as the large-current discharge characteristics or cycle life characteristics.